Mr Rotkovich left two other messages but eventually told his staff to watch for Holmes at the July 1 orientation and not to accept him into the club.

US police have recovered a computer belonging to Holmes, which could provide vital clues to how he planned the massacre, a senior officer said.

The computer was found inside his apartment, which officers only got access to late Saturday after clearing sophisticated booby traps rigged up inside, apparently intended to kill police arriving after the shootings.

"I'm told there was a computer inside the apartment and with the assistance of the FBI that computer will be completely analyzed," Aurora police chief Dan Oates told CBS talk show "Face the Nation."

"That may take some time. So we're hopeful that will yield some information," he added.

Mr Oates also revealed that, when Holmes was spotted behind the cinema after the shooting, his full body armour could easily have led police to think he was a member of a SWAT team.

"Without going into to too many details, there was one aspect of what he wore that did not fit what a SWAT officer might be wearing," he said.

"There was one particular piece of equipment that he had on him that was out of place and I am so proud of my officers that they spotted that right away and challenged him.

"I can't tell you in that chaos it is quite reasonable that an officer might have confused him for a SWAT officer, a heavily armed officer who was responding to the scene.

"So, kudos to my two cops that grabbed this guy and that sharp observation that they made immediately that led them to suspect him as being the suspect," he said.

Meanwhile, a six-year-old video has emerged showing Mr Holmes as a thin teenager wearing an oversized shirt and occasionally breaking into a slight, shy smile.

America's ABC News has aired the video showing Holmes speaking at a science camp at Miramar College in San Diego when he was 18.

His presentation is on "temporal illusion", which he defines as "an illusion that allows you to change the past".

He says he studies subjective experience, calling it "what takes place inside the mind, as opposed to the external world".

A woman who introduces Mr Holmes says he wants to become a researcher and make scientific discoveries.

She says his dream is to own a Slurpee machine, eliciting laughter from the audience and a smile from Mr Holmes.